The sonodynamically induced antitumor effect of a gallium-porphyrin complex, ATX-70, was evaluated in mice bearing colon 26. In order to find the optimum timing for the ultrasonic exposure after the administration of ATX-70, the ATX-70 concentrations in the plasma, skin, and tumor were measured and analyzed. Antitumor effect was estimated by measuring the tumor size. When used alone, ultrasound showed a slight antitumor effect, which became increasingly significant as the dose of ATX-70 was increased, while use of ATX-70 alone had no significant effect. At an ATX-70 dose of 2.5 mg/kg or higher, the average tumor size decreased to smaller than a half by three days after the ultrasonic exposure. This was smaller than a third of the size of the untreated tumors on the same day. From these results, it is concluded that ATX-70 significantly sensitizes tumors to ultrasound, demonstrating a synergistic antitumor effect.